Boolean difference acting like union (checked normals, doubles, and manifold)
Your treasure chest 113Mb blend file was to large for my machine to handle you may want to &quot;clean up&quot; the STL before you upload files. Do a &quot;Limited Dissolve&quot;. http://youtu.be/-OynHqYd_1I
Blender stackexchange has it's own file attachment system to help prevent files being &quot;lost / moved&quot; http://blend-exchange.giantcowfilms.com
Also are you going to 3D print this out? if so there's certain offsets when doing a Bool cut you need to take into account. Like making the cutting object about 0.2mm larger.
Are you using Blender 2.91? The &quot;Bool Tool&quot; using the Exact option may also help.
Here's an animated gif of the process using a box and your lid cutter.